Stair Stringer Calculator

Risers. Total rise ÷ your maximum riser height, rounded up to a whole number of risers. The riser height is then total rise ÷ risers, so every riser is the same. The IRC maximum is 7 3/4 in; enter a lower number for an easier stair.

Treads and run. Treads = risers − 1, because the top step lands on the upper floor and that floor is the last tread. Total run = treads × tread depth. Tread depth is the run of one step, nosing to nosing, so the nosing overhang is not added on top.

Stringer length. √(total run² + total rise²). The board to buy is that length plus your extra for trimming (12 in is a typical allowance, not a rule), rounded up to the next standard lumber length: 8 to 20 ft in 2 ft steps, per the Southern Forest Products Association’s specification guidelines. Longer stringers are a special order, and a flight that long is usually close to needing a landing.

Stair angle. arctan(riser height ÷ tread depth).

Number of stringers. (stair width − 1 1/2 in) ÷ maximum spacing, rounded up, + 1, so the outer stringers sit at the edges and no gap is wider than the spacing on center. The IRC sets no stringer spacing. The 18 in default is the American Wood Council’s DCA 6 deck guide maximum for cut stringers under 2× or 5/4 treads (Figure 29). Use your tread maker’s span if it is shorter.

Drop. Cut the bottom of each stringer down by one tread thickness so the first and top steps come out the same height as the rest. This assumes the stringer bears on the finished floor. If it bears on subfloor before the finish floor goes down, subtract the finish floor thickness from the drop.

Throat. The wood left below the notches is the board depth less riser × tread ÷ √(riser² + tread²). Dressed depths are 11 1/4 in for a 2 × 12 and 9 1/4 in for a 2 × 10. DCA 6 calls for at least 5 in on deck stairs (Figure 28), and limits a cut stringer to 6 ft of horizontal span between supports, so a longer deck stair needs a post or a landing under the stringers.

Code check. Riser 7 3/4 in max (R311.7.5.1), tread 10 in min (R311.7.5.2), 3/8 in max variation between the largest and smallest riser or tread in a flight, width 36 in min (R311.7.1), headroom 6 ft 8 in (R311.7.2), 151 in max rise per flight (R311.7.3), and a handrail on one side at 4 or more risers (R311.7.8). These are 2021 IRC numbers. The 2024 IRC moved them to R318.7 and the handrail rules to R320 without changing the limits. Your local code may amend them, and commercial stairs follow the IBC.

Price. Material is stringers × your board price, treads × your tread price and, for closed risers, risers × your riser board price. Labor is risers × your per-step rate, or your lump sum for the stair. Nothing is priced with numbers you did not enter.

Pricing the whole job? The construction estimate template has material, labor, equipment and subcontractor sections with overhead and margin on top.